As a part-and-parcel to Sam's work, I'd recommend Charles Freeman's "The Closing of the Western Mind" (Knopf, 2003). While not to recap the whole book, where did Christian "orthodoxy" come from? For almost three centuries after Christ's death there were a plethora of proto-Christian beliefs - many that would be unrecognizeable by today's "standards" - and tomes were filled with cross-condemnation of heretical beliefs. As Freeman broadly concludes, it was Constantine who essentially forced the Church to define ortodoxy at Nicea in 325, not because he had any particular interest in Christianity (or was even Christian), but wanted political order in an empire coming under increasing strain. As a reward for the Church ending its internal disputes, tax exemption and access to generous state funding was given to the "orthodox," thereby allowing the Church to grow by leaps and bounds. (At the beginning of the 4th century, about 5% of the inhabitants of the Roman Empire were Christian; about half by the end of the century after it had become "state sponsored.")

At any rate, what Christians came to view as some form of immutable truth or revalation from God as the dictats contained in the Nicean Creed ("One Holy and Apsotolic Church, the Trinity", yada, yada) is nothing more than what a bunch of guys in a room - under severe political pressure - came to put down on a piece of paper. (While not covered in Freeman's book, much the same could be said of Judaism under Cyrus, a whole other topic.) Finally, for those who really want to get into more detail on how "Christianity" evolved in the first three centuries - why certain gospels are now included and others not, etc - Bart Ehrman's "Lost Christianities" (Oxford, 2003) is most interesting.